if you don't want them to rattle just get some rubber washers from any hardware store and install them between every metal to metal contact area. (i.e. between plate and frame and between plate and vehicle.)
edit: I lost all my pics of my car when my hard drive crashed so I will try to get my dad to take some and sent them to me so I can show you what mine looks like. Although, it is pretty basic. It's just all silver with BMW printed on the bottom
1970 BMW 2002
Driven by skill, not money
"Real BMW's have round tail lights"